\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\tl_new:N \l__dai_temp_tl

\keys_define:nn { daikeys } {
    foo .tl_set:c = { l_dai_ \tl_use:N \l__dai_temp_tl _tl },
}

\keys_define:nn { daikeys } {
    foo .code:n =
     {
       \tl_clear_new:c { l_dai_ \tl_use:N \l__dai_temp_tl _tl }
       \tl_set:cn { l_dai_ \tl_use:N \l__dai_temp_tl _tl } {#1}
     },
}

\NewDocumentCommand \MyCommand { m  m } {
    \tl_set:Nn \l__dai_temp_tl { #1 }
    \keys_set:nn { daikeys } { #2 }
}
\ExplSyntaxOff

\begin{document}
\MyCommand{bar}{foo=foobar}

\ExplSyntaxOn
\l_dai_bar_tl
\l_dai__tl
\ExplSyntaxOff

\end{document}
